(1) Field of the Invention
The invention relates to a rotary wing aircraft, e.g. a rotary wing aircraft rotary wing aircraft with a shrouded tail rotor.
(2) Description of Related Art
A ducted tail rotor (e.g. a Fenestron®) is a rotor placed inside a tail of a rotary wing aircraft. The part of the tail containing the rotor is called shroud. The shrouded tail rotor is rotating inside a duct placed in the shroud. The shroud has a substantially toroidal geometrical shape, with the rotor rotation axis being the axis of the toroidal shape.
The shroud delays the rotor stall, if the blade tip is sufficiently close to the duct, with a blade clearance of 0.4%-1% of the shrouded tail rotor radius, thus increasing the rotor efficiency. The shroud allows the rotor induced flow to create as well regions of under-pressure on a collector side of the duct thus generating additional thrust in the same direction of the shrouded tail rotor's thrust, so that roughly half of the total anti-torque is generated by the rotor and the rest by the shroud. In forward flight condition the shrouded rotor anti-torque is at its minimum because most of the anti-torque generation function is taken over by the vertical fin mounted on top of the shroud and the bumper at its bottom.
So basically the shroud is designed for hover and sideward flight conditions, whereas in forward level-flight, descent and climb the vertical fin and the bumper produce the requested anti-torque. Numerical analysis and flight testing of known shrouds of rotary wing aircrafts highlight that any side-forces produced by these shrouds and the associated vertical stabilizing surfaces, i.e. fin and bumper in forward flight conditions are similar in value. Yet the shrouds cause significantly more drag than the vertical fins. This implies that the aerodynamic efficiency, i.e. side force over drag, of the shrouds is much poorer than that of the vertical fins.
Typically the trailing edge surface of the shroud is blunt and sharp in order to improve the rotary wing aircraft handling quality during sideward flight, e.g. lateral flight, when the rotary wing aircraft has a side slip angle beta of 90 or 270 deg. or quartering flight, when the rotary wing aircraft has beta angles of 45, 135, 315 deg. etc., thanks to the defined airflow separation on the sharp left and right edges of this trailing edge surface. Yet, since the blunt trailing edge surface is quite wide it highly contributes to the overall shroud drag during forward flight. Additionally this wide trailing edge surface is essentially perpendicular to the rotary wing aircraft mid-plane (y=0); leading to somewhat identical aerodynamic effective surfaces between left and right hand side of the shroud of the tail rotor. In other words, the x-z-projection surfaces of the left and the right hand side of the shrouds of the state of the art are identical.
Since the vertical fins are typically defined through at least one airfoil featuring high lift over drag and lift variation over angle of attack values, said vertical fins are highly effective, in contrast to the shrouds which are designed to first of all enclose the shrouded tail rotor as well as to improve the shrouded tail rotor thrust performance in hover and sideward flight by generating additional side force due to the suction effect of the shrouded tail rotor induced flow on the shroud's surface.
The document US2012/0138731 A1 describes a blunt trailing edge of a shroud of a shrouded tail rotor comprising sinusoidal protuberances in the shroud longitudinal direction and orthogonal to the H/C mid plane.
US2012/0061522 A describes a shape of an airfoil including a three dimensional wave pattern in order to improve aerodynamics as well as reducing nuisance noise.
U.S. Pat. No. 5,102,067 A describes an integrated rotary wing aircraft empennage structure with a shrouded anti-torque unit in order to improve overall H/C performance with an overall integration of the vertical and horizontal stabilizing mean into the aircraft structure.
The document U.S. Pat. No. 5,131,604 describes a rotary wing aircraft having a tail boom and an empennage structure including a shroud, a vertical stabilizer, and a horizontal stabilizer, a ducted fan anti torque device. Airflow duct surfaces have an axis and are configured for mounting internally in the shroud with said axis substantially perpendicular to the medial plane of the shroud. The airflow duct includes a divergent duct portion downstream of and contiguous to the inlet. An outlet terminates the divergent duct portion and has a curved lip configuration that includes a first constant radius lip segment, a second constant radius lip segment, and intermediate lip segments of variable radius intermediate said first and second constant radius lip segments. A support strut has an elliptical configuration and includes non-radial support strut extending non-radially with respect to the airflow duct axis.
Other prior art documents were considered while preparing the application, i.e.: DE2204619, FR2600036, EP0680872, EP2460720, U.S. Pat. Nos. 3,750,982, 4,809,931, 5,588,618, WO9828187 and WO2014025252.
The state of the art low curvature shroud leading edges can feature unsteady flow detachment and reattachment in lateral and quartering flight of a rotary wing aircraft.